The invention relates to a method for assessing a terrain surface for suitability as a landing area or taxi area for aircraft.
Synthetic vision systems are being increasingly used for landings under restricted visibility conditions. These are intended to provide the most realistic possible representation of the actual circumstances in the landing zone and, in this context, particularly on the ground surface. The data sources used are either databases containing map material (e.g. DTED elevation databases) and/or active 3D sensors, which survey the landing zone before and/or during the landing operation. These data are then numerically processed and visually displayed. The application of such synthetic vision systems has been proposed for helicopter landings under brownout/whiteout conditions or under otherwise severely reduced visibility conditions. In this context, it is crucial that besides the pure, most precise possible geometric properties of the landing zone, such as the inclination thereof relative to the horizontal plane, any obstacles are also represented correctly and with an exact position.
Similar requirements apply to automatic landing systems, both for helicopters and for fixed-wing airplanes. In this case, there is also a need to obtain precise information about the landing zone or the landing strip (position in space and quality) and to guarantee freedom from obstacles.
German patent document number DE 10 2004 051 625 B4 outlines a general approach to the general solution to the brownout problem. This involves the use of a high-resolution 3D sensor to produce a 3D representation of the landing zone during the landing approach. After the brownout situation is entered, no further new data are recorded. On the contrary, the existing data are presented as a synthetic view of the landing zone with the helicopter movement.
“Beyond Brownout”, Aviation Week & Space Technology, Apr. 6, 2009, pages 44, 45 reports on a project in which fully and semi-automatic landing under brownout conditions was realized. This project involved a scanning millimetre wave radar being developed and tested as a sensor. For the synthetic view produced, data from a terrain database were combined with the data from the millimetre wave radar (the relevant method is described in European patent document number EP 2 182 326 A1). If the sensor detected obstacles in a landing zone, the helicopter was able to change its landing spot automatically or with the assistance of the pilot.
PCT Publication number WO 2008/018906 A2 describes a further method for producing a synthetic view to assist the pilot. In this case, a terrain database forms the basis of the synthetic view. This terrain database has its interpolation points verified by real-time sensor data from one or more distance sensors, and possibly modified.
The known methods have the drawback that available 3D sensors are not capable of achieving sufficiently accurate spatial resolution for the landing area. A limiting effect is produced in this context firstly by the beam divergence of the active sensors (radars and ladars) used and secondly by the accuracy of the distance measurement. Added to this are errors by the navigation system, the data from which are used for geo-referencing the 3D data. Unlike in the case of topographic measurement flights, in which the interleaving of the measurement beams and the perpendicular measurement angle determine the resolution exclusively using the distance resolution of the 3D sensor used, the beam divergence also significantly influences the achievable spatial resolution in the application being considered here, on account of the dragging, acute measurement angle during the final approach.
In commercially available “high-resolution” radar systems, the resolution limit for the detection of objects at close range should be set to approximately 1 m; in commercially available ladar systems, it is between 30 cm and 50 cm on account of the smaller beam divergence. Both resolutions appear to be critical for a safe landing, however. An undetected obstacle of barely a meter would with a high level of probability result in a catastrophic collision for a landing helicopter. Even a rock or trench with a size or depth of 40 cm could result in a landing helicopter overturning.
The known systems are based on taking the individual measured values from the 3D sensors and computing the ground surface of the landing zone by approximation or interpolation and then displaying it. The measurement inaccuracies described above on account of the spatial resolution of the 3D sensors mean that synthetic representation of the ground area by individual area elements requires the measured values within a discrete, local area element either to be averaged or to be usefully approximated using other filters or methods. This results in oversimplification of the measured data, but this is the only way in which the represented ground area becomes sufficiently noise-free. The problem is largely independent of the shape of the area elements. Generally, rectangular or triangular area elements are used which may be either regular, i.e. having a constant grid size, or irregular, i.e. having a changing grid size. A further problem is the size of such local area elements, which is crucial for determining the requisite computation power of the real-time graphics processors required for the presentation. Typical local area element sizes range between one metre and a few metres of edge length at close range below 200 m distance to the aircraft. Smaller obstacles and ground irregularities could only be represented using significantly smaller local area elements. However, this would mean significantly greater computation complexity, which is limited by the real-time capability of the available hardware.
A thesis by DYLAN KLOMPARENS: “AUTOMATED LANDING SITE EVALUATION FOR SEMI-AUTONOMOUS UNMANNED AERIAL VEHICLES”, INTERNET CITATION, 20 Aug. 2008 (2008-08-20), pages 1-129, XP002569083, found on the Internet: URL: http://scholar.lib.vt.edu/theses/available/etd-08192008-231631/unrestricted/Thesis.pdf, describes a method in which a 3D sensor in the form of a stereo camera is used to examine the landing zone of a vertical take-off and landing unmanned aerial vehicle (VTOL UAV) for suitability as a landing area. The three-dimensional measured values are used to derive individual area elements for the examined landing zone, and the orientation of the area elements in space is determined. Differences in the spatial orientation of adjacent area elements are used as a measure of the roughness of the terrain, with the thus ascertained measure of the roughness additionally being combined with further statistical properties of the three-dimensional measured values as a plausibility check.
Exemplary embodiments of the present invention provide a method that can be used to obtain up-to-date and highly accurate information about the position and quality of the landing zone or taxi area of the aircraft.
The method according to the invention allows insights about the local properties of the surveyed landing zone or taxi area to be obtained despite the limited spatial resolution of the 3D sensors used by oversampling (i.e., producing 3D data which repeatedly cover the terrain to be surveyed in multiple measurement cycles, e.g., multiple scans or, in the case of a flash ladar, in multiple snapshots) and statistical evaluation. These local properties relating to the ground quality then result in classification, which determines whether or not this area is suitable for landing or as a taxi area. The result can be displayed in a synthetic view of the surveyed landing zone or taxi area as an additional feature—either as a marker, as a texture or as colour coding—of each corresponding local area element. The thus obtained property of each local area element can likewise be transmitted to an automated landing system, which uses this information to plan the landing.
The method according to the invention is particularly suitable for application for rotary-wing aircraft, particularly in order to cope with the critical states of the “brownout” or “whiteout” when taking off or landing on sandy or snow-covered ground.
The principle on which the invention is based is universal, however, and can likewise be used for fixed-wing aircraft. In this case, the intended landing strip for the fixed-wing aircraft is analyzed and rated in a similar fashion in terms of its quality and particularly for the presence of an obstacle. The generally greater distance between the aircraft and the ground area to be surveyed in comparison with a rotary-wing aircraft has no influence on the strategy according to the invention.
Furthermore, the method presented here can be used not only for rating the landing zone or landing strip in a final approach of an aircraft, but also for rating taxiways and parking areas on airfields; that is to say while the aircraft is moving on the ground prior to takeoff or after landing.
All known radar-based or laser-based sensor types can be used as the 3D sensor. Laser radars and high-resolution radars, particularly millimetre wave radars, are particularly suitable.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.